GATE 2002:

In 2’s complement addition, overflow

(a) is flagged whenever there is carry from sign bit addition

(b) cannot occur when a positive value is added to a negative value

(c) is flagged when the carries from sign bit and previous bit match

(d) None of the above

oops its B here ..but just remember the following conditions for overflow 1> if the sign bit numbers are different then overflow never occurs if both are same both 0 or both 1 then overflow occurs iff the result of adding it is different .

Ex.. 0001+0111 — > 1000 both Lsb are 0 but result is 1 so here 4 bit overflow..

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2) second condition is xor of the left two carry bits is 1 then its overflow.. this condition is specifically for twos complement …

and to crosscheck .. just perform the addition or subtraction in decimal… if it can’t fit into the given figits… ie for above example its 4 bit addition… and ans requires more than that.. 5 bits…